The invention relates generally to engines, and more particularly, to a system and method for controlling peak cylinder pressure and/or substitution ratio of an engine, for example, a dual fuel engine.
In a compression-ignition engine, such as a diesel engine, a fuel injection system injects fuel (e.g. diesel fuel) into compressed air within each of the engine cylinders to create an air-fuel mixture that ignites due to the heat and pressure of compression. Unfortunately, engine efficiency, power output, fuel consumption, exhaust emissions, and other operational characteristics are less than ideal. In addition, conventional techniques to improve one operational characteristic often worsen one or more other operational characteristic. For example, attempts to decrease specific fuel consumption often cause increase in various exhaust emissions. Vehicle exhaust emissions include pollutants such as carbon monoxide, nitrogen oxides (NOx), particulate matter (PM), and unburned hydrocarbons (UHC) generated due to imperfect combustion of fuel within the combustion chamber. The amount of these pollutants varies depending on the fuel-air mixture, compression ratio, injection timing, ambient conditions, and so forth.
In the oil and gas market and transportation sector, for example, the fuel bill is a significant contributor to the total cost of operation. The rapid expansion and abundance of natural gas in some areas of the world is driving a dramatic cost advantage of natural gas over diesel fuel, making natural gas an attractive fuel source. A dual fuel engine is based on a traditional diesel engine, with the addition of dual fuel specific hardware. When the engine is operating in dual fuel mode, natural gas is introduced into an intake system. Near the end of the compression stroke, diesel fuel is then injected. The diesel fuel ignites and the diesel combustion causes the natural gas to burn.
At higher brake mean effective pressure levels, when the substitution ratio of natural gas to diesel is increased, the engine is more susceptible to knock. Further, when the substitution ratio is increased, the allowable range of air to fuel ratio (AFR) between knock and misfire is reduced. When a fixed geometry turbocharger is used, it is difficult to control the airflow in an efficient manner to the engine.
There is a need for an enhanced system and method for controlling the operation of an engine.